Function Calling
OpenAI's name for tool use. Same concept, slightly different API surface.
Last updated: April 26, 2026
Definition
Function calling is OpenAI's implementation of tool use. The model emits a JSON object describing which function to call and with what arguments. As of GPT-5, function calling supports parallel calls (multiple tools in one response) and structured output guarantees (the model is constrained to your JSON schema). Functionally equivalent to Anthropic's tool use and Google's function declarations. The differences are mostly API ergonomics, not capability.
When To Use
When using OpenAI models. The mental model is identical to Anthropic tool use. Describe functions, model picks one.
Building with Function Calling?
I've shipped this pattern in real production systems. If you want a second pair of eyes on your architecture, that's what I do.